More than 100 members of the European parliament have signed a letter of support for two whistleblowers convicted by a Luxembourg court of leaking information about secret tax rulings to journalists.

Antoine Deltour and Raphaël Halet were both convicted of theft earlier this year after providing journalists with evidence that the rulings had helped multinational corporations slash their corporation tax bills to 1% or less.

Revelations from the so-called LuxLeaks prompted demands for reform of the international tax system following their publication in 2014 by media organisations, including the Guardian and the International Consortium of Investigative Journalists.

However, Luxembourg pursued criminal charges against Deltour and Halet with the support of their employer, the accountancy firm PwC, which had arranged the agreements.

The MEPs used the letter to call for greater protection for whistleblowers at national and international level. “We can only but agree with judge Marc Thill who stressed in his deliberations that both of you should be considered as ‘whistleblowers’ and that the facts which you exposed are of crucial public interest by virtue of their contribution to a more transparent and fairer tax system,” the letter reads.

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Tax rulings, also known as “comfort letters”, are a practice whereby national tax authorities issue guidance to companies on how their corporation tax will be calculated.

Although legal, the use of such rulings has become controversial following both the LuxLeaks reports and the revelation that Ireland allowed Apple to reduce its corporation tax liability for decades through its use of subsidiaries in the country. In 2014, the company paid a corporation tax rate of just 0.005%, according to Margrethe Vestager, the EU competition commissioner.

Fabio De Masi, the German MEP who organised the letter and vice-president of a committee investigating the Panama Papers, criticised the Luxembourg authorities for also appealing against the acquittal of Edouard Perrin, a French investigative journalist accused of helping Halet.

“It’s absolutely absurd that a journalist who covered Luxleaks is being brought to court,” said De Masi. “This is a fundamental threat to press freedom. It’s his job to cover these issues.”